Unit 2-Chemistry of Cooking

Question 1

During cooking processes, the proteins become denatured as the protein chains unwind .

Explain why the protein chains unwind.

Answer 1

Heat breaks hydrogen bonds between the side chains of amino acids.

Question 2

Many common flavours from different foods are caused by molecules within the foods called aldehydes and ketones.

Both these molecules contain which functional group?

Answer 2

Carbonyl (C=O)

Question 3

Describe how to identify an aldehyde:

1. From the name
2. From a structure

Answer 3

1. Names end in -al
2. Carbonyl group (C=O) is at the end of the carbon chain.

ALDEHYDE AT THE SIDE

Question 4

What steps are needed in order name an aldehyde?

Answer 4

1. Find the longest carbon chain and name it.
2. Number the carbon atoms in the chain so that the functional group has the lowest possible number. For aldehydes, since the carbonyl group is at the end of the chain, the carbon of the C=O is always on carbon number one.
3. Identify any branches joined onto the main chain and name them.
4. Identify each branch by a number indicating its position. Used Di = 2 branches, Tri = 3 branches, Tetra = 4 branches if needed.

Question 5

What are the systematic names of these aldehydes (alkanals)?

Answer 5

2,2-dimethylbutanal and butanal (not 1-methylpropanal)

Question 6

Describe how to identify a ketone:

1. From the name
2. From a structure

Answer 6

1. Names end in -one
2. Carbonyl group (C=O) is never at the end of the carbon chain.

Question 7

What steps are needed in order name a ketone?

Answer 7

1. Find the longest carbon chain and name it.
2. Number the carbon atoms in the chain so that the functional group has the lowest possible number.
3. Identify any branches joined onto the main chain and name them.
4. Identify each branch by a number indicating its position. Used Di = 2 branches, Tri = 3 branches, Tetra = 4 branches if needed.

Question 8

What are the systematic names of these ketones (alkanones)?

Answer 8

Butanone and 3-methylpentan-2-one

Question 9

Which chemicals can be used to tell aldehydes and ketones apart?

Answer 9

1. Acidified potassium dichromate solution

or

2. Fehling’s solution

or

3. Tollen’s reagent

Question 10

When heated with an oxidising agent, is it aldehydes or ketones who show any reaction?

Answer 10

Only aldehyde molecules will show any reaction when heated with an oxidising agent.

Question 11

What colour changes would be seen when an aldehyde is heated with?

1. Acidified potassium dichromate solution
2. Fehling’s solution
3. Tollen’s reagent

Answer 11

1. Acidified potassium dichromate solution

Orange → Green

2. Fehling’s solution

Blue → Brick red precipitate

3. Tollen’s reagent

Clear → Silver mirror precipitate